1. Field of the Invention
The present invention relates, generally, to apparatus for joining two structural sheets by heating a bonding material disposed between the juxtaposed edges of two metal sheets and, more specifically, to electrical heating apparatus for heating a bonding material to join two sheets together.
2. Description of the Art
The use of a heat curable, adhesive bonding material to join two juxtaposed structural elements or metal sheets is widely used in the automotive industry. Such a bonding technique is employed for doors, deck lids, hoods and like assemblies in which two metal sheets or panels are arranged in an edge overlapping manner with a suitable bonding material placed between the two sheets before the edge of one sheet is hemmed over the adjacent edge of the other sheet. Heat is then applied to the bonding material to cure the material and form a high-strength joint between the two structural elements or sheets.
It is well-known that the assembly of automotive vehicles involves high production rates. As such, electric induction coils have been employed to provide the necessary heat to quickly cure the bonding material. Such induction coils carry a high frequency electrical current which generates a magnetic field and causes heating of the metal sheets, which heat is conducted to and cures the bonding material disposed between the two sheets. Since the hemmed perimeter areas of doors, deck lids, hoods and the like can be quite large, it is common to employ such induction coils at only selected locations along the length of any hem so as to spot cure the bonding material only at the locations of the induction coils. The remainder of the bonding material is cured at a later time during the assembly process, such as when the automotive vehicle passes through a paint oven. It is also known to use induction coils around the entire perimeter edge or edges of an assembly to cure all of the bonding material at one time.
A typical prior art induction coil for bonding applications is shown in FIG. 1. In this typical arrangement, an induction coil 10 is positioned below an insulating material support 12. The support 12 receives and supports a hemmed edge 13 of an outer sheet or panel 14 and an inner sheet or panel 16. A strip of adhesive bonding material 18 is disposed between the outer and inner sheets 14 and 16 prior to the hemming operation. The passage of a high frequency electrical current through the induction coil 10 generates a magnetic field which induces heat in the metal sheets 14 and 16, which heat is conducted to and cures the bonding material 18 in the area of the induction coil 10.
FIG. 2 depicts another typical induction coil which is described in greater detail in U.S. Pat. No. 4,950,348. The induction coil 20 includes a generally rectangular-shaped coil assembly having a pair of outwardly extending legs 22, only one being shown in FIG. 2, which are connected to a source of electrical power, such as a transformer, not shown. A riser 24 and mounting base 26 are connected to the coil 20 to provide a suitable mount for the coil 20. Cores 28 are disposed about selected portions of the coil 20 for concentrating the magnetic field generated by the coil 20. In use, the prior art coil 20, shown in FIG. 2, is disposed on only one side of a hem joint between two overlapping sheets, such as the sheets 14 and 16 shown in FIG. 1.
The use of a single induction coil, as shown in the prior art arrangements of FIGS. 1 and 2, requires long heating times to cure the bonding material to a sufficient strength. Further, since the single induction coil is located on only one side of the hemmed joint of two sheets, more heat is generated at one sheet than on the other sheet resulting in an uneven heating of the bonding material.
FIG. 3 depicts yet another prior art induction coil used in bonding applications. In this arrangement, which is described in greater detail in U.S. Pat. No. 4,602,139, a lower fixture 30 attached to any suitable tool or support, not shown, carries a first or lower induction coil 32 which is mounted in a block of electrically insulating material 34. The lower fixture 30 also supports the hemmed joint formed between a lower outer sheet 36 and an inner disposed upper sheet 38, with a strip of adhesive bonding material 40 disposed therebetween. An upper fixture 42 supports a second or upper induction coil 44 which is also mounted in a block of electrical insulating material 46 in the upper fixture 42. The upper fixture 42 and the lower fixture 30 are movable with respect to each other so as to be separated from each other to permit the mounting or removal of the hemmed sheets 36 and 38 between the two fixtures.
In one mode of operation, the upper fixture 42 is lowered over the hemmed sheets 36 and 38 to place the second induction coil 44 above one surface of the hemmed edge of the sheets 36 and 38. The first or lower induction coil 32 is positioned substantially in line with the second coil 44, but is located below the hemmed edges of the sheets 36 and 38. The use of two induction coils 32 and 44 on opposite sides of the hemmed joint of the sheets 36 and 38 creates a more even heating of the bonding material 40 since heat is applied to the sheets 36 and 38 from both sides at the same time. However, the design of the tooling in this arrangement is more complicated since at least one of the fixtures, such as the upper fixture 42, must be movable with respect to the other fixture to permit the mounting and removal of the hemmed sheets 36 and 38 therebetween.
U.S. Pat. Nos. 5,365,041 and 5,442,159, assigned to the Assignee of the present invention, disclose an apparatus and method for heating adhesively bonded metal sheets utilizing an L-shaped coil. The L-shaped coil has a first leg which is disposed underneath the hemmed metal sheet joint and a perpendicular second leg located adjacent to the side edge of the hemmed joint in close proximity to the outer surface of the upper metal sheet.
This L-shaped coil, as compared to single side heating, causes the temperature of the upper sheet to more closely approximate that of the lower sheet. The temperature of the lower sheet is slightly hotter than the upper sheet due to the first coil leg being disposed in near proximity to the lower sheet; while the upper sheet is primarily influenced by the second leg of the coil which is not in as near proximity to the upper sheet as the first coil leg is to the lower sheet. Although the upper surface temperature is cooler than that of the lower sheet, the temperature difference between the two sheets is less than that obtained with single side heating thereby providing a better adhesive bond.
However, if the joined sheets have a lower surface which is convex, as it typical in an automotive body panel, the inclusive angle between the first and second legs of the coil must be greater than the preferred 90xc2x0 to facilitate loading of the panel onto the fixed coils. This inclusive angle adversely increases the temperature difference between the lower and upper sheets.
Thus, it would be desirable to provide a novel induction bonding apparatus and method which cures a bonding material to a higher strength than in prior art bonding apparatus. It would also be desirable to provide an induction bonding apparatus and method which creates a high strength, cured bond between two sheets or panels in less time than with previously devised heating apparatus for bonding applications. It would also be desirable to provide an induction bonding apparatus and method which uniformly heats both of the upper and lower surfaces of a hemmed adhesive joint.
The present invention is an induction heating coil for curing a bonding material disposed between overlapped or hemmed portions of two structural members or metal sheets by heating the bonding material to a cured state.
In one aspect of the invention, the induction heating coil includes a first element formed of a first inductive heating surface adapted to be disposed adjacent to one of the two metal sheets and the bonding material disposed therebetween, and a second element having a shape complementary to the shape of the first element and spaced from the first element. The second element forms a second inductive heating surface adapted to be disposed adjacent to an opposite one of the two structural members or metal sheets and the bonding material disposed therebetween.
In the invention, the second element is preferably equally spaced from the first element along the length of both of the first and second elements. Preferably, the first and second elements are parallel. In one aspect, the first and second elements are spaced in parallel planes.
The first and second elements may have any shape, including a straight shape between opposed ends or a curved shape, such a convex or concave shape in one plane, between opposed ends. This enables the first and second elements to follow the peripheral edge of an irregularly shaped hemmed structural assembly.
In one aspect of the invention, conductive elements connect the first and second elements into a single turn coil. Preferably, the conductive elements comprise first and second end legs which are respectively joined to first and second end-to-end legs forming the first element. Third and fourth end legs are connected to opposed ends of the second element. The first, second, third, and fourth end legs are electrically connected, such as by interconnecting legs.
First and second spaced risers are connected to and extend from the first and second legs of the first element. First and second base members are connected to the first and second risers, respectively, for connecting the first element to a source of high frequency electrical current.
In one aspect of the invention, the first and second risers, the first and second legs of the first element and the first and second end legs are coplanar. Likewise, the third and fourth end legs and the second element are coplanar with each other.
Coolant flow paths are formed in the first and second elements, including the first and second risers, the first, second, third, and fourth end legs and the interconnecting legs between the first, second, third, and fourth end legs. The coolant flow paths are preferably formed by constructing the first and second elements, the risers and the end legs of hollow, tubular members.
Magnetic field concentrating members may be disposed about the first and second elements for concentrating the magnetic field generated by the first and second elements through the structural members or metal sheets disposed between the first and second elements.
The induction coil of the present invention provides advantages over previously devised induction coils used to cure a bonding material between two overlapped or hemmed structural members or metal sheets. The essentially parallel arrangement of the first and second elements of the present induction coil which are connected as a single turn coil may be configured in a variety of shapes to follow the shape of the peripheral edge of the structural members or metal sheets disposed therebetween. Thus, while the first and second elements are preferably parallel and spaced equidistantly from each other between opposed ends, the first and second elements may take any linear or non-linear, irregular shape between the opposed ends to closely follow the shape of the peripheral or hemmed edge of the structural members or metal sheets disposed therebetween; while at the same time providing uniform heating of both of the upper and lower surfaces of the hemmed joint between the structural members or metal sheets.